<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>内部css</title>
        <style type="text/css">
            /*标签选择器*/
            div {
                background-color:red;
                width: 500px;
                height: 100px;
            }
            /*id选择器*/
            #first-div {
                background-color:blue;
                width: 400px;
                height: 200px;
            }
            /*class选择器*/
            .bg-div {
                background-color:black;
                width: 300px;
                height: 300px;
            }
        </style>
    </head>
    <body>
        <div>
            标签选择器
        </div>
        <div id="first-div">
            id选择器
        </div>
        <div class="bg-div">
            class选择器
        </div>

    </body>
</html>